This invention deals in the art of optical image correlators and more particularly with an improvement therein respecting the filtering of the output correlation signal. Heretofore, numerous types of optical image correlators have been known in the art and the invention herein deals particularly with such correlators as those shown generally in U.S. Pat. No. 3,496,290, to R. H. Smith; No. 3,564,126, to Richard F. Koch; No. 3,424,937, to W. L. Steiner; and No. 3,423,624, to W. L. Stiner. These prior art patents have been assigned to Goodyear Aerospace Corporation of Akron, Ohio, the assignee of the invention presented herein.
While optical image correlators may be of numerous types, the general structure of such correlators includes a photo emissive cathode upon which an optical input signal is cast. Electrons are emitted from the cathode and directed, under control of deflection coils, to a storage grid which maintains an electrical charge indicative of a reference image. Of course, a collector grid may be interposed between the storage grid and the photo cathode for control of secondary emission. The output signal of the optical image correlator is directly related to the degree of optical correlation existing between the optical input signal and the reference optical image maintained upon the storage grid. The deflection of the electron stream passing from the photo cathode to the storage grid causes the input optical signal or image to be nutated upon the stored optical image with a maximum output signal being evidenced from the correlator when the input image and stored image are in registration with each other.
With reference now to FIG. 1, it can be seen that an optical image correlation system is designated generally by the numeral 10 and is shown as including an optical image correlator 12 of the nature discussed hereinabove and more particularly as set forth in any of the aforementioned prior art patents. As shown in FIG. 1, and as set forth in the referenced prior art, the output of the optical image correlator 12, which is indicative of the degree of correlation existing between the optical input image signal and a stored optical image, is passed to the circuitry 14 containing preamps and noise cancellation circuitry. The output of the circuit 14 is passed to a filter 16 which emits a filtered correlation signal indicative of the correlation function performed by the optical image correlator 12. As discussed earlier, the correlation function is performed at 12 by means of controlling deflection coils maintained therein and interposed between the photo cathode and storage grid. The electrons passing across the deflection coils, as known in the art, are nutated to effectuate the correlation technique. This nutation is controlled by means of the deflection coils control circuitry 22 which feeds its output signal to the deflection coils; such signal being a function of the correlation signal and a variable amplitude sinusoidal signal controlled by a ramp generator 18 which produces a ramp function output signal. As the output of the generator 18 linearly changes its amplitude, the corresponding amplitude of the sinusoidal output signal of the circuit 20 changes and correspondingly the deflection of the electron stream is controlled by the deflection coils control circuitry 22.
In utilizing the system shown in FIG. 1, it has been found that as the ramp generator 18 causes the circuits 20, 22 to affectuate a nutation cycle of the deflection coils of the correlator 12, the output signal of the correlator 12 changes in frequency with respect to the information content thereof. This is readily appreciated in that the varying amplitude nutation signal applied to the coils via the circuit 22 results in a varying degree of movement of the electron stream (representing the optical image input signal) with respect to the fixed image maintained upon the storage grid. This change in relative motion between the input image and the stored image results in a frequency content shift of the correlation signal emitted from the correlator 12 during the nutation cycle.
In order to circumvent the aforementioned problems inherent with velocity variations of the electron stream and the frequency content shift of the correlation signal, filtering circuits have been devised for receiving and filtering the correlation signal. The known filters generally function as a multiple step search filter with each step providing a constant filter corner for a portion of the nutation signal. Such filters require sequential switching to the appropriate filter section at particular points along the ramp function output of the generator 18. Of course, each section of the filter has a frequency response characteristic uniquely adapted to its related portion of the ramp function. However, to achieve optimum filtering, an infinite number of sections of the filter would be necessary so that the filter can congruently trace the ramp function rather than in a stepping fashion. Indeed, utilizing the sectioned-filter approach, a trade-off must be made between filter circuit complexity and output signal integrity. Further, it has been found that multiple step search filters inherently create noise problems within the system due to the transients which occur upon switching to each step.
Consequently, it is an object of the instant invention to provide an optical image correlator tracking filter circuit wherein the frequency response of the filter traces the ramp function controlling the coil nutation.
Yet another object of the invention is to present an optical image correlator tracking filter circuit wherein the inherent trade-offs necessary when using sectioned filters may be alleviated.
A further object of the invention is to present an optical image correlator tracking filter circuit wherein the noise generation problems of prior art filters are non-existent.
Still another object of the invention is to present an optical image correlator tracking filter circuit which is reliable in operation, simplistic in design, and readily conducive to implementation with presently existing optical image correlators and utilizing state-of-the-art elements.
These objects and other objects which will become apparent as the detailed description proceeds are achieved by an optical image correlator circuit, comprising: an optical image correlator receiving an optical image input signal, converting said input signal to an electron stream and nutating said stream as it approaches a storage grid upon which a reference image is maintained as a distributed charge, said correlator producing an output signal indicative of the correlation between the input signal and the reference image; deflection coils interposed within said correlator for nutating said electron stream; a deflection coil control circuit interconnected with and controlling said deflection coils for linearly regulating the rate of nutation of said electron stream; a filter circuit interconnected between said correlator and said deflection coil control circuit for receiving and filtering the output signal of the correlator, said filter circuit having a characteristic frequency response which linearly varies with, and is under control of, an output of said deflection coil control circuit.